540i Sporadic Coolant temperature

[Waynebrady]

Hi Guys,

Haven’t posted here in a while and lost access to my main account :/. Anyways, I have an odd issue where every once in a while maybe 1 out of 20 drives. I’ll notice my temperature gauge start to rise, if I turn on the heat it will immediately return to dead center on the temperature gauge and stay there.

At first I thought it was an air bubble in the system but I’ve driven the car probably 6,000 miles since it started happening so it it should have worked it’s way out. 4,000 of those miles It didn’t happen at all, now that it’s getting colder here in Michigan it’s happening again, rarely.

I’m thinking either a faulty thermostat or maybe a water pump on its way out? The car has no issue with warming up in a timely matter so the T stat isn’t stuck open. And besides once in a blue moon the car stays perfectly at temp. My two theory’s are a water pump on its way out. Maybe the bearing is beginning to go bad and by turning the heat on I essentially use the heater core as a secondary radiator to make up for the lack of flow? Or the thermostat gets temporarily stuck closed, although I’m not sure how the cabin heat would interact with that.

Any ideas or someone who’s experienced similar symptoms?

Notes: new radiator and overflow tank 6k miles ago, aftermarket overflow

2003 540I/6 M-Sport 119xxx

[Ed CT]

So, this started 6000 miles ago, and the radiator and expansion tank (NOT an overflow tank) were replaced 6000 miles ago?

I'm thinking there's a small leak somewhere where air get sucked in during engine cool down, creating an air bubble. Sometimes the bubble gets worked out of the system, and sometimes not, causing the temp gage spike. The expansion tank could be suspect if it's aftermarket. Is it a BMW tank or other brand? Best to go with OEM for that important cooling system part.

[R Shaffner]

You've checked the coolant level when cold, right? And make sure it's full, or maybe a little low, and not too full.

Next time the engine is cold do one more thing: start the engine with the hood open and the expansion tank cap off. It's OK if the coolant level drops a little with the engine running, but it shouldn't drop too much. If it does, then you have an issue with how the coolant system is working, and you probably too little coolant. If you need to add coolant while the engine is running, I'd go ahead and bleed the system bu letting air out of the bleed screw after the thermostat opens (and starts to send hot coolant to the radiator).

This info would have helped me as a new 540 owner:

https://www.bimmerforums.com/forum/s...ding-Explained

[Waynebrady]

I’m leaning towards replacing the expansion tank with OEM. But i still don’t understand the relationship the heater core has with the issue? Does it force the coolant to flow when it’s turned on?

It’s definitely not leaking any coolant, but I suppose it’s possible air is getting introduced to the system. I would think air would cause excessive pressure in the system which would force some coolant out.

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Shaffner, yes the coolant level is correct when stone cold. However, when the engine is started the level drops significantly.

[MotorMouth93]

The expansion tank isn't going to cause overheating if its not leakinglol. It's just a tank, it sits there.

I'd start by replacing the thermostat. Sounds like if anything it might be getting stuck closed. The 540i cooling system basically bleeds itself and a lot of people get hung up on imagined details.

[R Shaffner]

I’m leaning towards replacing the expansion tank with OEM. But i still don’t understand the relationship the heater core has with the issue? Does it force the coolant to flow when it’s turned on?

It’s definitely not leaking any coolant, but I suppose it’s possible air is getting introduced to the system. I would think air would cause excessive pressure in the system which would force some coolant out.

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Shaffner, yes the coolant level is correct when stone cold. However, when the engine is started the level drops significantly.

How far is significantly? If the red indicator drops an inch or two, that's fine, if it's dropping a lot more, than can be (probably is) because you have a breach (leak) in the system that might be hard to spot. That's one of the biggest and most annoying issues with these systems that use an expansion tank at the same height as the radiator. They're designed to maintain a full radiator while having a half-full expansion tank. But sometimes they can fake you out and make you think the system is full when the radiator is only half-full (at the same level as the coolant in the tank).

I'll give you an example. When I was still getting to know my first 540, many years ago, it developed a slow leak where the small tube along the top of the radiator connects to the bleed fitting on the expansion tank. The leak was slow and the coolant came out while driving, so it was hard for me to spot. As pressure would build in the system, it would lose coolant. And when it cooled down it would suck air back into the top of the radiator. As time passed, more coolant was lost and more air was pulled back into the radiator.

I started getting inconsistent temp readings, like you are. And I started to get low coolant warnings as soon as I started the car from cold. But when I stopped the engine and checked the coolant level, it showed full. So I thought I must have a bad coolant level sensor or loose wiring.

Finally, after the engine got too hot for comfort once, I check the system thoroughly. I found the leak, fixed it, bled the system properly, and solved the problem for good.

In short, if the system is losing coolant when hot, it will suck air back in when cold, and that air will collect in the top of the radiator. Motormouth93 is right about the system's being somewhat self-bleeding, but only when there isn't a breach in the system, and not too much air.

Here's another simple test you can try: When the engine is cold, before you start it, take the cap off the expansion tank, note the level of the red indicator, and then loosen (or remove) the bleed screw. If the system is full, properly bled, and you don't have another opening in the system, then loosening the bleed screw will let a little air back in the top of the radiator, which will let coolant flow back into the expansion tank. Then the indicator will rise and might show it's too full. But it isn't. In that case you can close it up and drive it normally. The system will refill the radiator and push air back into the expansion tank.

However, if the coolant level in the tank does NOT rise when you loosen the bleed screw, or if it only rises a little, then your system wasn't properly filled and bled. In that case you should refill it and bleed it. And after you bleed it you should let it warm up with the cap on the and the bled screw in, so the system pressurizes. Then look for leaks.

[TiesTorN]

bad aux fan maybe?

[jicaino]

Another possible cause could be the heater valve. Those solenoids could be leaky when heater is off and not leaking when you turn on the heater.